Recent years, data quantity transmitted on transmission lines connecting information processing apparatuses has increased. In order to cope with the data quantity increase on transmission lines, it is effective to transmit data in a high frequency signal. For transmission of a high frequency signal, a high frequency clock signal is used.
In a data transmission circuit which uses a high frequency clock signal, peaking developing on a data signal waveform causes problems such that the data signal waveform is distorted, a voltage greater than an allowable voltage is applied to a circuit, and so on. On a conventional transmission line, a high frequency component in a transmission signal is likely to be attenuated. Accordingly, when attenuation of a transmission signal is substantial, the transmission signal needs to be amplified up to an amplitude high enough for demodulation.
FIG. 17 is a diagram illustrating data transmission waveforms in a conventional binary data transmission. The data transmission waveforms in FIG. 17 illustrates exactly the behavior of a flip-flop, in which input data DIN is sampled according to a clock signal CLK. A waveform of an output signal DOUT in a conventional binary data transmission is created by delaying a data input DIN temporally, and the output signal DOUT created in such a way propagates on a transmission line. The output signal DOUT reaches the reception side as a rounded waveform the high frequency component of which is, in the propagation process, attenuated due to frequency response characteristics of the transmission line. The output signal DOUT is sampled according to a clock signal in a similar manner to the above-described behavior of a flip-flop and restored to the original data.
On a conventional transmission line, a transmission signal is attenuated as the signal propagates through the transmission line, causing the transmitted waveform thereof to be rounded and rising edges and falling edges thereof to become less apparent. For example, when the input data DIN in FIG. 17 is rounded in the propagation process and peaking develops between two peaks (signals of “1”), the two peaks may join together, making it difficult to demodulate an output signal DOUT accurately. Accordingly, it is preferable to amplify the transmission signal by an amplification device installed on the transmission line or in a receiving unit before the transmission signal becomes too much attenuated to be demodulated.
FIG. 18 is a block diagram illustrating a data transmission system 10 disclosed in Patent Literature 1 (Japanese Patent No. 3719413). The data transmission system 10 of Patent Literature 1 includes a sending unit 110 and a receiving unit 120. The sending unit 110 is connected to the receiving unit 120 by a reference signal transmission line 140 through which a reference signal REF is transmitted and a modulation signal transmission line 150 through which a data signal DAT is transmitted.
In the data transmission system 10 of Patent Literature 1, the reference signal REF and modulation signal DATA are transmitted from the sending unit 110, which inputs input data DIN and a clock signal CLK, to the receiving unit 120 through the reference signal transmission line 140 and the modulation signal transmission line 150, respectively. The receiving unit 120 receives the reference signal REF and modulation signal DATA and outputs an output signal DOUT by demodulating the signals.
FIG. 19 is a time chart of the data transmission system of Patent Literature 1. In the data transmission system of Patent Literature 1, information indicated by “0” and “1” in the amplitude direction in the input data DIN is modulated to information in the phase direction of the reference signal REF and modulation signal DATA in the sending unit 110. The transmitted signal is, after the phase differences are converted to negative pulses S (phase lag signal) and R (phase lead signal) by a phase detection unit in the receiving unit 120, restored to a data string of “0” and “1” by an RS latch (S: Set, R: Reset).
FIG. 20 is a time chart illustrating the phase differences between the reference signal REF and the modulation signal DATA illustrated in the time chart in FIG. 19 in an accentuating manner. As illustrated in FIG. 20, the input data DIN is modulated so that the phase of the modulation signal DATA (solid line) leads with respect to the reference signal REF (dotted line) when the input data DIN is 0, and the phase of the modulation signal DATA (solid line) is delayed with respect to the reference signal REF (dotted line) when the input data DIN is 1.
A system which transmit information in input data DIN by using phase differences of a modulation signal DATA with respect to a reference signal REF as disclosed in Patent Literature 1 is also disclosed in Patent Literature 2 (Japanese Patent No. 3982517) and Patent Literature 3 (Japanese Patent Application Laid-Open No. 2011-77791).
In an ordinary transmission line, attenuation of a signal increases as a higher frequency is used, which causes a problem such that it becomes difficult to transmit a signal as a higher frequency is used for the transmission. In an ordinary data transmission system, there is also a problem such that signal transmission becomes difficult when peaking develops. In particular, in a general transmission in which information is represented by binary amplitude, because a signal includes a plurality of frequency elements, interference is likely to be caused between consecutive codes when the amplitude of the signal increases at a particular frequency due to peaking. Although it may be possible to insert an amplifier on the transmission path to eliminate effects of interference between codes, power consumption will increase due to the signal amplification.
With the data transmission systems of Patent Literatures 1 to 3, transmission characteristics such that interference between codes does not cause ill effects even when higher frequency is used may be achieved. However, even with the data transmission systems of Patent Literatures 1 to 3, inserting a general amplifier on the transmission path causes extra power consumption to be required.
Accordingly, an object of the present invention is to provide a data transmission system which makes it possible to suppress attenuation of a waveform of data signal without extra power consumption in a high-speed transmission by a high-speed clock.